The invention relates to a scanning speed modulating (VM) circuit which is used to improve sharpness of a display image in an image display apparatus using a cathode ray tube (CRT) such as television receiver, video monitor, or the like and, more particularly, to a scanning speed modulating circuit which is useful when a peripheral image is sharpened in an image display apparatus using a cathode ray tube whose display screen is flat.
In an image display apparatus using a cathode ray tube such as television receiver, video monitor, or the like, there is a case where a scanning speed modulating circuit is used to sharpen a display image. According to the scanning speed modulating circuit, a steep change point of a video signal is detected and a scanning speed of an electron beam of the cathode ray tube is modulated at the change point, thereby emphasizing dark/light states of an outline portion of the image and enabling the image to be sharply seen.
FIG. 1 shows a conventional example of a scanning speed modulating circuit for modulating a beam scanning speed of such a cathode ray tube and correcting an outline. Reference numeral 1 denotes a video signal terminal; 2 a video amplifying unit; and 3 a cathode ray tube (CRT) to which an electron beam is scanned by a deflecting coil DY.
Reference numeral 4 denotes a scanning speed modulating circuit unit comprising: a differentiating circuit 4a for detecting a change point of a video signal; a waveform shaping circuit 4b for shaping a differentiating waveform; a differentiating amplifier 4c; and an amplification output unit 4d. An output of the amplification output unit 4d is supplied to a scanning speed modulating coil 5 attached to a neck portion of the CRT 3. A video signal from the video amplifying unit 2 is supplied to a cathode K1 of the CRT 3.
In a television receiver having such a scanning speed modulating circuit, for example, when a video signal as shown in FIG. 2A corresponding to an image comprising a white portion and a black portion is supplied, a current of a signal waveform differentiated at a change point of the video signal as shown in FIG. 2B can be supplied to the speed modulating coil 5. Thus, an intensity of a synthetic magnetic field of the deflecting coil DY for scanning an electron beam in the horizontal direction and the scanning speed modulating coil 5 changes as shown in FIG. 2C. A moving speed of the electron beam is subjected to a speed modulation such that it is accelerated in the + side direction for a usual horizontal scanning speed and soon decelerated in the xe2x88x92 side direction as shown in FIG. 2D.
Thus, a luminance of an image which is displayed on the CRT 3 changes so as to become blacker at a point where the speed rises and become whiter at a point where the speed decreases as shown in FIG. 2E. As shown in FIG. 2F, a portion just before a vertical line which rises from black to white is displayed blacker and a portion just after the rising is displayed whiter, thereby enabling an outline portion of the image to be sharply seen.
However, even when the speed modulation as mentioned above is performed, there is a case where the sharpness cannot be sufficiently improved with respect to a peripheral image. The reasons of it will now be described hereinbelow.
A raster of a cathode ray tube is formed by scanning the electron beam emitted from the cathode K1 of the CRT 3 onto a fluorescent surface of the CRT. In this instance, the nearer a scanning position approaches a peripheral portion of the raster, the longer a flying distance of the electron beam is, so that a deflecting sensitivity of the peripheral portion of the raster is higher than that of the center portion of the raster. Therefore, a phenomenon such that the image of the peripheral portion of the raster is extended in the scanning direction as compared with the image of the center portion of the raster occurs.
To prevent such a phenomenon, a correction called an S-character correction such that an inclination near the scan start portion and an inclination near the scan end portion of a scanning signal having a saw-tooth-shaped waveform are made gentler as compared with an inclination of the center portion, thereby making the deflecting sensitivity of the peripheral portion.of the raster and that of the center portion identical is performed.
The deflecting sensitivity regarding the speed modulation will now be considered. By reasons similar to those mentioned above, a deflecting sensitivity due to a deflecting magnetic field generated by the scanning speed modulating current as shown in FIG. 2B also rises in the peripheral portion of the raster of the CRT. Therefore, if a level of a speed modulation signal is set so as to obtain a proper sharpness in the center portion of the raster, the speed modulation is excessively performed in the peripheral portion of the raster, so that a problem of deterioration of the sharpness contrarily occurs. Particularly, in the peripheral portion of the raster, since the electron beam enters a normal of the fluorescent surface at an angle of inclination, a focusing deterioration easily occurs, so that the deterioration of the sharpness in the peripheral portion of the raster becomes typical. Further, when the display screen of the CRT is flat, in the peripheral portion of the raster, since the electron beam enters the normal of the fluorescent surface at a larger inclination angle, the deterioration of the sharpness becomes more remarkable.
To solve such a problem, a technique such that the level of the speed modulation signal in a scanning region of the peripheral portion of the raster where the deflecting sensitivity is high is reduced to thereby decrease an outline correction amount has been proposed in, for example, JP-A-5-244449.
In the prior art disclosed in the above Official Gazette, a correction signal is modulated in a parabolic shape so that a correction amount of the speed modulation in the center portion of the raster is smaller than that of the peripheral portion of the raster.
However, if the correction amount of the speed modulation is merely changed in the parabolic shape as shown in the prior art disclosed in the above Official Gazette, problems as will be explained hereinbelow remain.
The focusing deterioration in the CRT does not continuously occur from the center portion of the raster toward the peripheral portion of the raster but tends to suddenly occur in the peripheral portion near the outer edge of the raster. In most of the regions of the raster, a good focus is obtained. Therefore, it is desirable to improve the sharpness of the image by performing a speed modulation to most of the regions of the raster.
However, in the prior art disclosed in the above Official Gazette, since the correction amount of the speed modulation is merely sequentially reduced along the parabolic waveform from the center portion of the raster toward the peripheral portion of the raster, if the correction amount of the speed modulation is set so that a proper speed modulation is performed in the center portion of the screen, the correction amount is insufficient in a region near the peripheral portion of the raster. Therefore, there is a problem such that the sharpness of the image cannot be sufficiently improved in a region near the peripheral portion of the raster.
Although the deflecting sensitivity increases almost in proportion to a tangent function and the focusing deterioration also corresponds to it, since the correction amount of the speed modulation is changed along the parabolic waveform in the prior art disclosed in the above Official Gazette, there is a problem such that the change of the correction amount of the speed modulation of the amount necessary to eliminate the focusing deterioration and the change of the correction amount of the speed modulation which is actually supplied do not correspond to each other and the optimum correction cannot be performed.
When an influence that the speed modulation exerts on a picture quality of the image is now examined, a phenomenon such that when the speed modulation is strongly performed, generally, a vertical white line becomes thinner than the inherent width and a black line becomes thicker exists. For example, in the case where a black character exists in the image, the black character becomes thick by performing the speed modulation and, in the case where a white character exists in the image, the black character becomes thin by performing the speed modulation.
Although the focusing deterioration occurs in the peripheral portion of the screen as mentioned above, when the thickening of the black character in the peripheral portion of the screen is compared with the focusing deterioration in the peripheral portion of the screen, the deterioration of the picture quality due to the focusing deterioration becomes a problem. Therefore, when a character existing in the peripheral portion of the screen is a black character, even if the thickening of the black character occurs, it is desirable to improve the picture quality by performing the speed modulation.
However, in the prior art disclosed in JP-A-5-244449, since the correction amount of the speed modulation is sequentially reduced along the parabolic waveform from the center portion of the raster toward the peripheral portion of the raster, the correction amount of the speed modulation for the peripheral portion of the screen decreases and the deterioration of the picture quality in the peripheral portion of the screen cannot be prevented. When the correction amount of the speed modulation for the peripheral portion of the screen is increased, if a white character exists in the peripheral portion of the screen, a problem of the occurrence of a blur of the white character occurs.
It is, therefore, an object of the invention to provide a scanning speed modulating circuit of an image display apparatus which can properly prevent a deterioration of a picture quality in a peripheral portion of a screen.
Another object of the invention is to provide a scanning speed modulating circuit of an image display apparatus which can perform a speed modulation by an enough correction amount even in a region near a peripheral portion of a screen and can obtain an image whose picture quality is improved in most of regions of the screen.
Still another object of the invention is to provide a scanning speed modulating circuit of an image display apparatus which can improve a picture quality due to a speed modulation even in a peripheral portion of the screen.
To solve the above problems, according to the invention, there is provided a scanning speed modulating circuit of an image display apparatus, in which by modulating a scanning speed of an electron beam of a cathode ray tube in accordance with a video signal that is supplied to the cathode ray tube, sharpness of an image which is displayed on the cathode ray tube is improved, comprising:
correction signal generating means for generating a speed modulating correction signal to modulate the scanning speed of the electron beam on the basis of the video signal;
correction signal control means for controlling a level of the speed modulating correction signal by a reference signal having a predetermined waveform that is changed synchronously with a scan of the electron beam; and
reference signal generating means for generating a signal, as a reference signal, of a waveform such that it decreases along a curve that is analogous to a deflecting sensitivity of the cathode ray tube in a peripheral portion of a screen of the cathode ray tube.
The correction signal generating means can generate the reference signal of the waveform such that it decreases along a curve which is almost flat in a center portion of the screen of the cathode ray tube and whose both ends are analogous to the deflecting sensitivity.
The reference signal generating means can include: a square circuit which inputs a saw-tooth wave signal synchronized with the scan of the electron beam; and a limiter for limiting a level of an output of the square circuit.
The square circuit can be constructed by a variable gain multiplying circuit whose two input terminals are connected in common.
The scanning speed modulating circuit further comprises: detecting means for detecting a level according to an average luminance of the video signal in portions corresponding to at least right and left peripheral portions of the screen of the cathode ray tube; and maximum value detecting means for comparing the level detected by the detecting means with a level of the reference signal generated by the reference signal generating means and supplying the signal of the higher level to the correction signal control means.
According to the invention, the correction signal generating means generates the reference signal of a waveform such that it decreases along a curve which is almost flat in the center portion of the screen of the cathode ray tube and in which, in portions on the outside from almost 60xc2x0 as a deflection angle of the cathode ray tube, both ends are analogous to the deflecting sensitivity.
As mentioned above, according to the invention, since the signal of the waveform such that it decreases along a curve that is analogous to the deflecting sensitivity of the cathode ray tube in the peripheral portion of the screen of the cathode ray tube is used as a reference signal to perform the speed modulation, a blur of a thin white vertical line in the peripheral portion of the screen which is caused by the scanning speed modulation, particularly, the deterioration of a fine white character can be prevented.
According to the invention, since the speed modulation is performed on the basis of the reference signal of the waveform such that it decreases along the curve which is almost flat in the center portion of the screen of the cathode ray tube and whose both ends are analogous to the deflecting sensitivity, the speed modulation can be performed even in a region near the peripheral portion of the screen by the enough correction amount, and an image whose picture quality is improved in most of the regions of the screen can be obtained.
According to the invention, since the speed modulation in the peripheral portion of the screen can be selectively performed strongly or weakly in accordance with the average luminance of the video signal in the portions corresponding to the right and left peripheral portions, particularly, in the case where the average luminance is high because there is a black character or the like in the peripheral portion of the screen, its sharpness can be raised.